Mention docs in README, add badge for docs

README.rst

@@ -21,6 +21,8 @@
 	   :target: https://github.com/psf/black
 .. image:: https://coveralls.io/repos/github/Happy-Algorithms-League/python-gp/badge.svg?branch=master
 	   :target: https://coveralls.io/github/Happy-Algorithms-League/python-gp?branch=master
+.. image:: https://readthedocs.org/projects/ansicolortags/badge/?version=latest
+	   :target: https://happy-algorithms-league.github.io/hal-cgp/
 
 Cartesian genetic programming (CGP) in pure Python.
 
@@ -36,8 +38,6 @@ Figure from Jordan, Schmidt, Senn & Petrovici, "Evolving to learn: discovering i
 
 .. long-description-end
 
-A simple example of CGP applied to a symbolic regression problem can be found in `examples/example_evo_regression.py`.
-
 .. installation-start
 ============
 Installation
@@ -77,6 +77,8 @@ The adventurous can install the most recent development version directly from ou
 Basic usage
 ===========
 
+For the detailed documentation, please refer to [https://happy-algorithms-league.github.io/hal-cgp/](https://happy-algorithms-league.github.io/hal-cgp/).
+
 Follow these steps to solve a basic regression problem:
 
 1. Define an objective function.